The high-flying Londoners, who were vying for a ninth straight victory, were handed huge impetus in the bid to prolong their perfect record on 18 minutes, as defender Andy Butler was sent off for a two-footed lunge on Romain Vincelot.
Russell Slade's men struggled to adapt to their numerical advantage initially, with the O's failing to replicate the type of form which has seen them net 23 times in their opening eight fixtures.
However, the hosts began to capitalise on their advantage and it was not long before the visitors were punished, with in-form hitman David Mooney firing his team to a 1-0 lead.
Mooney was whiskers away from bagging his and his team's second of the afternoon minutes after the restart, running on to a clever pass from Dean Cox, before dispatching an angled drive which whistled wide of the target.
It would prove a costly miss, as the Saddlers restored parity through Craig Westcarr, who headed home the equaliser on 71 minutes.
The hosts threw everything at Dean Smith's men in an attempt to prolong their perfect start to the League One season, but a red card for Vincelot in the closing stages extinguished any hopes of finding a winner.